MXS-2495 Cleanup GWBUF interface

All GWBUF macros that address a single link in a chain are now
simple wrappers for equivalent gwbuf_link-functions.

Next step is to drop the macros and replace their use with calls
to the functions.

server/core/test/test_buffer.cc

@@ -39,6 +39,12 @@
 #include <maxscale/buffer.hh>
 #include <maxscale/hint.h>
 
+/*< Return the byte at offset byte from the start of the unconsumed portion of the buffer */
+#define GWBUF_DATA_CHAR(b, byte) (GWBUF_LENGTH(b) < ((byte) + 1) ? -1 : *(((char*)(b)->start) + 4))
+
+/*< Check that the data in a buffer has the SQL marker*/
+#define GWBUF_IS_SQL(b) (0x03 == GWBUF_DATA_CHAR(b, 4))
+
 /**
  * Generate predefined test data
  *

server/core/test/test_modutil.cc

@@ -593,7 +593,7 @@ void test_large_packets()
     {
         GWBUF* buffer = gwbuf_append(create_buffer(0x00ffffff), create_buffer(i));
         mxb_assert(gwbuf_length(buffer) == 0xffffffUL + i + 8);
-        GWBUF_RTRIM(buffer->next, 1)
+        GWBUF_RTRIM(buffer->next, 1);
         GWBUF* complete = modutil_get_complete_packets(&buffer);
         mxb_assert_message(buffer, "Incomplete buffer is not NULL");
         mxb_assert_message(complete, "The complete buffer is not NULL");

server/modules/protocol/MySQL/mariadbbackend/mysql_backend.cc

@@ -729,7 +729,7 @@ static inline bool collecting_resultset(MySQLProtocol* proto, uint64_t capabilit
  */
 static inline bool not_ok_packet(const GWBUF* buffer)
 {
-    uint8_t* data = GWBUF_DATA(buffer);
+    const uint8_t* data = GWBUF_DATA(buffer);
 
     return data[4] != MYSQL_REPLY_OK
            ||   // Should be more than 7 bytes of payload